mem-cache: Adopt a more sensible cache class hierarchy

This patch changes what goes into the BaseCache and what goes into the
Cache, to make it easier to add a NoncoherentCache with as much re-use
as possible. A number of redundant members and definitions are also
removed in the process.

This is a modified version of a changeset put together by Andreas
Hansson <andreas.hansson@arm.com>

Change-Id: Ie9dd73c4ec07732e778e7416b712dad8b4bd5d4b
Reviewed-on: https://gem5-review.googlesource.com/10431
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>

src/mem/cache/write_queue_entry.cc

@@ -139,7 +139,7 @@ WriteQueueEntry::checkFunctional(PacketPtr pkt)
 }
 
 bool
-WriteQueueEntry::sendPacket(Cache &cache)
+WriteQueueEntry::sendPacket(BaseCache &cache)
 {
     return cache.sendWriteQueuePacket(this);
 }